The present invention relates generally to the field of combustion and flue gas cleanup methods and apparatus and, in particular, to a new and useful apparatus and method for removing mercury from flue gases generated by combustion, through the use of an alkaline sorbent.
In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers and waste to energy plants. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, mercury remains in the vapor phase at relatively low temperatures and does not condense onto fly ash particles. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg++) or elemental (Hg0) form and each is affected differently by subsequent downstream pollution control equipment.
Most of the recent efforts to capture and remove mercury from the flue gas produced by coal-fired units have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
The subject of mercury emissions by the utility and waste to energy industries is a new area being investigated by both the DOE and EPA.
Approximately 75% of existing coal-fired power plants are not equipped with wet flue gas desulfurization (WFGD) systems. These systems most often control particulate emissions with electrostatic precipitators (ESP""s) and baghouses. With possible mercury emissions regulation for the electric power industry pending, it is imperative to have cost-effective mercury capture technologies available for those power plants lacking WFGD systems.
It is known to inject limestone in dry powder form into the flue gases in the upper furnace cavity of a boiler for the purpose of capturing SO2 from the flue gases. A discussion of systems using this process can be found in U.S. Pat. Nos. 5,795,548 and 5,814,288 to Madden et al. These systems or processes are also referred to as Enhanced Limestone Injection Dry Scrubbing processes/systems, or E-LIDS systems(trademark), a trademark of The Babcock and Wilcox Company. Please refer to FIG. 1.
For the E-LIDS(trademark) processes or systems, a particulate collection device is located downstream of the air heater to remove particulate matter from the flue gases exiting the boiler. Any one of several known types of particulate separation techniques may be employed for this purpose, including inertial impaction separators, fabric filters (baghouses) and ESP""s. Flue gases exiting from the particulate collector then pass through a dry scrubber where they are contacted by a slurry containing calcium hydroxide. Calcium is introduced in stoichiometric molar ratios of calcium to sulfur much greater than 1.0 and usually about 2.0 mole/mole. The high molar ratios are necessary to achieve good reactions between the calcium and sulfur present in the flue gases.
Additional SO2 removal can take place in a dry scrubber located downstream of the particulate control device, followed by a final particulate collector in which coal flyash, spent sorbent and unreacted sorbent particles are collected. A baghouse is preferred as the final particulate control device because of the additional SO2 removal it yields as the flue gases pass through the filter cake on the filter bags. Thus, the E-LIDS(trademark) process or system combines sorbent injection, dry scrubbing and fabric filtration.
It has been discovered that the E-LIDS(trademark) process also has the effect of removing 95% of the total amount of mercury present in the furnace system. Surprisingly, it was discovered that 82% of the mercury removal occurred using the sorbent injection and first particulate collector alone.
It is an object of the present invention to provide a cost efficient solution for reducing mercury emissions in flue gases that is easily retrofit into existing power plant systems.
Accordingly, one aspect of the present invention is drawn to a mercury removal system for removing mercury from a flue gas generated in utility and waste to energy combustion systems having a boiler and a stack, comprising: particulate removal means for separating and removing particulate matter containing mercury from the flue gas, the particulate removal means located between the boiler and the stack; and sorbent injection means for providing an alkaline sorbent in one of powdered and slurried form to at least one location upstream of the particulate removal means in the power plant, the alkaline sorbent being provided in a stoichiometric molar ratio of calcium to sulfur in a range of about 0.001 mole of an alkaline earth or an alkali metal/mole sulfur and 1.0 mole of an alkaline earth or an alkali metal/mole sulfur. The alkaline sorbents are injected into a power plant system at one or more locations and at stoichiometric molar ratios of and alkaline earth or an alkali metal to sulfur of less than 1.0 to remove at least between about 40% and 60% of the mercury content from power plant emissions. Small amounts of alkaline sorbents are thus injected into the flue gas stream at a relatively low rate. A particulate filter is used to remove mercury-containing particles downstream of each injection point used in the power plant system.
Under certain circumstances, it may be desirable to use a combination of both an alkaline earth sorbent and an alkali metal sorbent to accomplish mercury removal according to the present invention.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.